Electrochemical potential measurements are commonly used to determine solution pH, other selective ion activities, ratios of oxidation and reduction activities, as well as other solution characteristics. A pH/ion selective electrode/oxidation reduction potential meter (hereafter referred to as a pH/ISE/ORP meter) is typically a modified voltmeter that measures the electrochemical potential between a reference half-cell (of known potential) and a measuring half-cell. These half-cells, in combination, form a cell, the electromotive force (emf) of which is equal to the algebraic sum of the potentials of the two half-cells. The meter is used to measure the total voltage across the two half-cells. The potential of the measuring half-cell is then determined by subtracting the known potential of the reference half-cell from the total voltage value.
The measuring half-cell typically includes an ion selective material such as glass. The potential across the ion selective material is well known by those of ordinary skill in the art to vary in a manner that may generally be described by the Nernst Equation, which expresses the electrochemical potential as a logarithmic function of ion activity (thermodynamically corrected concentration). A pH meter is one example of a pH/ISE/ORP meter wherein the activity of hydrogen ions is measured. The definition of pH is the negative logarithm of the hydrogen ion activity and is typically proportional to the measured electrochemical potential.
One example of a pH/ISE/ORP meter is disclosed in U.S. Published Application No. 2011/0048971, filed Aug. 26, 2010 the contents of which are incorporated by reference in their entirety. This meter includes an elongate outer housing having first and second longitudinal ends. The outer housing may be formed from glass. A measuring half-cell received in the outer housing includes a stem glass tube extending along the length of the outer housing from adjacent the first end to adjacent the second end, and a pH glass membrane connected to the end of the stem glass tube adjacent the second end of the outer housing. A solution ground assembly received in the outer housing includes an electrical conductor received in a tubular non-electrically conductive sleeve extending along the length of the housing from adjacent the first end to adjacent the second end of the outer housing. Longitudinal end margins of the stem glass housing and the non-electrically conductive sleeve of the solution ground assembly are received in and generally fixedly secured to respective first and second seals adjacent to the corresponding first and second longitudinal ends of the outer housing. The first and second seals are generally fixedly secured in the outer housing.